1. Field of the Invention
The present invention relates to a telephone set, and more particularly to noise suppression for improved speech-quality.
2. Prior Art
Recently, a portable telephone set has been widespread with progressed mobile radio communication, and people have come to easily talk over the telephone at any place as well as specific places such as a telephone booth, etc. This is an important feature of the mobile radio telephone system. However, a noise problem arises in cases where the telephone is used in noisy surroundings.
In Japanese Patent Laid-open Publication No. SHO 63-191447, a noise suppression telephone set is disclosed. As shown in FIG. 1, the conventional telephone is provided with a noise detector 30 and a microphone 40 for detecting a noise level of the surroundings. The noise detector 30 outputs a detected noise level signal S20 to a receiver circuit 34, a filter circuit 37, and a ringing controller 39. According to the noise level signal S20, the gains of the receiver circuit 34 and the ringing controller 39 are changed, and the frequency characteristics of the filter circuit 37 are modified.
When an incoming call occurs at a line terminal 32, the ringing controller 39 generates speaker 31 to ring. Since the gain of the ringing controller 39 is changed according to the noise level signal automatically controlled in volume.
After the circuit has been established, receiving an incoming signal from a hybrid circuit 33, the receiver circuit 34 outputs a received acoustic signal S22 to a telephone receiver (or a speaker) 38 which transforms it into sound waves. Since the gain of the receiver circuit 34 is changed according to the noise level signal 820, the received acoustic signal S22 is automatically controlled in volume.
On the other hand, a telephone transmitter (or a microphone) 36 outputs a transmitting acoustic signal S23 to the filter circuit 37 comprising a high-pass filter. The cut-off frequency of the high-pass filter is changed according to the noise level signal S20 so that noise frequencies lower than the cut-off frequency, the noise having frequency characteristics similar to 1/f noise, are stopped to remove them from the transmitting acoustic signal S23. The transmitting acoustic signal S23 passing through the filter circuit 37 is sent to the line terminal 32 through a transmitter circuit 35 and the hybrid circuit 33.
In addition to the telephone transmitter 36, however, the above-mentioned telephone set necessitates the microphone 40 for detecting the surrounding noise. Furthermore the microphone 40 must be placed at a position apart from the telephone transmitter 36 so as to detect background noise only. Therefore, employing the conventional noise suppression system, it would be difficult to make the telephone set miniature and lightweight. Particularly, for a mobile telephone set, it is very important to make the telephone body small and light.